Wear-resistant rubber product and a method of making same

ABSTRACT

A wear-resistant rubber product adapted to be used as, for example, a lifter in grinding mills, comprises at least two prefabricated parts (11, 12) interconnected with one another by means of a binder layer having a thickness of at least 2 mm and consisting of in situ cast and cured polyurethane. The wear-resistant rubber product is manufactured by placing the prefabricated cured rubber parts (11, 12) at a distance of at least 2 mm from one another and by introducing, preferably injecting a reacting urethane rubber composition with excess isocyanate groups into the gap (13) to cause said prefabricated parts on either side of said gap to be contacted subtantially simultaneously by the said urethane rubber composition.

Wear-resistant rubber products are used to a large extent in the miningindustry, especially in chutes and, primarily, in mills for grindingore. In such mills, use is made of a wear-resistant lining consisting ofshell plates and so-called lifters which extend into the drum mill andstir the charge within the rotating mill drum and which therefore aresubjected to an exceptionally high wear. In view hereof, the liftersmust be replaced on more than one occasion during the life of the milllining or, strictly speaking, the shell plates. Normally, the wornlifters are replaced by new ones, and the worn lifters frequently arescrapped. The worn-out lifters usually still carry large quantities ofrubber since the lifters must be replaced before the grinding capacitywhich is dependent upon the lifter geometry, has decreased to anunacceptably low level. In order to reduce waste of lifter material, ithas therefore been suggested, for instance in U.S. Pat. No. 3,191,875and Swedish patent specification No. 335,460, gradually to raise thelifters in relation to the shell plates, thereby to use up as muchrubber as possible. It has also been tried to recap rubber lifters byusing recapping methods well known in connection with the recapping ofmotor vehicle tires, the strength requirements necessitating thetechnique of hot recapping.

However, the recapping methods used in the tire industry are not readilyadapted to the recapping of lifters because the worn rubber lifter mustbe thoroughly buffed and washed and given a primer coating and becausevulcanization must be carried out for a considerable period of timeunder heat and pressure in expensive vulcanizing moulds of steel inorder to produce a bond of sufficient strength. Furthermore, some of theknown recapping methods require access to special equipment. Theapplication of pre-vulcanized treads to used carcasses requires carefulshaping (abrading) of the rubber surface of the used detail to make itconform to the shape of the new vulcanized rubber detail which isvulcanized to the old rubber detail.

It therefore is an object of the present invention to provide a newwear-resistant rubber product which has the properties of the originalproduct and which, besides, is readily manufactured. Furthermore, theinvention comprises a novel method of making this product.

According to the invention, a wear-resistant rubber product which is tobe used as, for example, a lifter in grinding mills thus comprises atleast two prefabricated parts interconnected by means of a binder layerof in situ cast and vulcanized polyurethane having a thickness of atleast 2 mm (preferably at least 5 mm). The wear-resistant rubber productis made by placing the prefabricated vulcanized rubber parts at adistance of at least 2 mm (preferably at least 5 mm) from one anotherand filling the gap with a reacting urethane rubber composition withexcess isocyanate so that the prefabricated parts on either side of thegap are contacted substantially simultaneously by said urethane rubbercomposition. The characteristic features of the invention are stated inthe claims.

The wear-resistant rubber product may be either a new product or arecapped product in which parts of a used corresponding wear-resistantrubber product have been connected with at least one prefabricatedvulcanized wear-resistant rubber body by means of the in situ cast andvulcanized urethane rubber layer, said prefabricated and vulcanizedrubber body having a profile adapted to that of the future product.Since the conditions of prefabricating this rubber body are far morereadily controlled, a high-quality wear-resistant rubber body can beobtained. Hot recapping by vulcanizing an uncured rubber compound to anold and worn product in a vulcanizing mould always necessitates acompromise between on one hand the vulcanization of the newly suppliedrubber compound and thus the properties and adhesive power of saidcompound in the vulcanized state and, on the other hand, the risk ofovercuring or scorching of the rubber material of the old and wornproduct to be recapped.

When the invention is applied to the manufacture or recapping of milllifters, the product may be formed of at least two parts, one of whichcomprises a holder and the other consists of a prefabricated curedwear-resistant rubber body of a profile suitable for the future lifter.For the recapping, the part comprising the holder consists of a wornlifter, the surface of which is prepared by cleaning and, if necessary,also by buffing. Such a worn lifter may comprise a metal component withattachment means and a rubber part vulcanized thereto. The prefabricatedpart or parts used for the manufacture may consist of extruded ormoulded cured wear-resistant rubber bodies which, however, must berelieved of their vulcanization skin by some simple surface treatment,such as sawing or cutting. On the other hand, it is not always necessaryto buff the rubber bodies, and no primer need be applied. Thus, thecured wear-resistant rubber bodies and the worn lifter can be processedwith equipment less expensive than that used in, for example, thecomplete or partial recapping of tires.

During manufacture, the prefabricated parts preferably are placed on acasting substrate in such a manner that the minimum spacing betweentheir surfaces to be interconnected will be 2 mm, preferably 5 mm. Inthis manner, a gap is formed, the ends of which are closed to form anupwardly open mould cavity into which the reacting urethane rubbercomposition is then introduced, preferably injected. The minimum spacingof 2 mm, preferably 5 mm, is conditioned by the requirement that itshall be possible for the reacting urethane rubber composition safely toenter and fill out the gap between the bodies and to contact the bodiessubstantially simultaneously.

The rubber parts to be joined together may have room temperature whencontacted by the hot urethane rubber composition. On the other hand, itis often convenient to dry and preheat the prefabricated parts beforethe urethane rubber composition is cast because such drying andpreheating reduces the risk of bubble formation within the in situ casturethane rubber composition. Moreover, it was found that one can be sureto obtain satisfactory strength values if the prefabricated parts arepreheated. The urethane rubber composition preferably is introduced at atemperature of 75°-150° C. The upper limit is conditioned by the factthat the time or pot life during which the compounded urethane rubbercomposition can be used will be too short if the casting temperatureexceeds 150° C.

The invention will be described in more detail in the following,reference being had to the accompanying drawing in which FIGS. 1 and 2diagramatically illustrate two possible procedures in the manufacture ofa wear-resistant rubber product according to the present invention.

FIG. 1 shows a casting mould 10 in which a worn lifter 11 and aprefabricated wear-resistant rubber body 12 have been placed lying ontheir sides, such that a gap 13 having a minimum width of at least 2 mmis formed therebetween. The gap is being filled with a reacting urethanerubber composition from an injection nozzle 14 which is moved along thegap and in which the components of the urethane rubber composition aremixed immediately before the composition leaves the nozzle. Theinjection nozzle preferably is inserted into the gap, thereby to causethe liquid urethane rubber composition to rise and gradually to expelthe air within the gap and to contact both rubber bodies substantiallyat the same time.

FIG. 2 shows the manufacture of a new wear-resistant rubber bodyaccording to the present invention, the components of which have notpreviously been used. In this case, a sectional metal element 15 hasbeen placed in the center of the bottom of a casting mould 16. Twoprefabricated, cured wear-resistant rubber bodies 17, 18 have beenrigged up in the casting mould by means of small spacer members (notshown) provided at the ends thereof and, if necessary, also at one ormore points along the length of the rubber bodies so that an invertedY-shaped gap 19 is formed between the bodies 17, 18 and the sectionalmetal element 15. A needle-shaped nozzle 20 is used for filling the gap19 with the liquid reacting urethane rubber composition.

The invention will now be illustrated in more detail, reference beinghad to the following Examples.

EXAMPLE 1

In this Example, use was made of a worn lifter, the rubber part of whichwas formed of wear-resistant rubber based upon styrene-butadiene rubberand having a hardness of 60° Shore A. The worn lifter is cleaned andslightly buffed on its surface. The lifter had a length of 1 m and awidth of 2 dm. The lifter and a prefabricated sectional rubber elementof the same rubber type were placed overnight in a warming cupboardhaving a temperature of 110° C. The bonding surface of the prefabricatedsectional rubber element was conditioned by removal of a surface layer.The lifter was laid on its side on a plastic fabric(polytetrafluoroethylene) on a casting table. The sectional rubberelement was placed alongside of the lifter, such that the minimumdistance between this surface and the curved worn surface of the lifterwas at least 2 mm. The ends of the resulting gap between the lifter andthe sectional rubber element were sealed by pressing mould side wallsagainst the lifter and the rubber sectional element. A reacting urethanerubber composition was then injected into the gap by means of the nozzleof a mixer/injector that was inserted in the gap in order to fill saidgap while expelling the air. In this manner, the two rubber surfaceswere contacted with the reacting urethane rubber compositionsubstantially simultaneously, which is of essential importance to thereliability of the bond.

For the urethane rubber composition, the following two-component systemwas used:

Prepolymer

100 parts by weight polyester prepolymer having about 6.5% of availableisocyanate groups (MDI, i.e. methylene-bisphenylene-diisocyanate)

Curing agent

33.6 parts by weight hydroxyl-terminated polyester

3.7 parts by weight 1,4-butanediol

This recipe implies a curing agent content of 95%, based upon theavailable isocyanate groups.

The two rubber details had a temperature of 90°-100° C., and theurethane rubber composition injected into the gap had a mixingtemperature of 110° C. After the gap had been filled, the componentswere left on the casting table until the urethane rubber composition hadsolidified and obtained sufficient strength to allow handling of thelifter and the sectional rubber element attached thereto. The productwas then stored at room temperature for 72 hours.

From the lifter and the sectional rubber element vulcanized theretotransverse test pieces having a thickness of 25 mm were taken bysectioning. The pieces or samples were then subjected to adhesion tests,so-called peeling tests. The peel strength was found to be 26.5 kN/m,the rupture occuring in the styrene butadiene rubber material, not inthe joint between the urethane rubber and styrene butadiene rubbermaterials.

EXAMPLE 2

Example 1 was repeated, and both rubber components were buffed. The peelstrength was found to be somewhat lower, 24.8 kN/m, and the ruptureoccured in the styrene butadiene rubber material. This shows that it isnot necessary to buff the surfaces in order to establish adhesionbetween polyurethane and rubber, and that a cut surface (Example 1) issufficient. However, the vulcanization skin of the prefabricatedsectional rubber element must be removed.

EXAMPLE 3

Example 1 was repeated, except that no drying and preheating in awarming cupboard occured, which means that both rubber components hadroom temperature. The peel strength was 14.6 kN/m, and the ruptureoccured in the joint between the rubber materials. This test shows thatan acceptable strength is obtainable also with cold rubber, but thathigher strength values will be obtained if the rubber components arepreheated and dried.

EXAMPLE 4

In this Example, worn lifters and prefabricated sectional rubberelements of the same material and having the same dimensions as thelifter and the sectional rubber element of Example 1 were used. As inExample 1, the worn lifters were cleaned and slightly buffed, and asurface layer of the sectional rubber elements was removed to exposefresh rubber. The lifters and the sectional rubber elements werepreheated overnight in a warming cupboard having a temperature of 100°C. Each lifter was then placed in a shallow casting mould which stood ona casting table and had such dimensions that the lifter in uprightposition, i.e. with the metallic holder facing downwardly, fittedclosely in the casting mould, the edges of which protruded beyond therubber part of the worn lifter. In a first test, the lifter was coveredwith the urethane rubber composition stated in Example 1, and thesectional rubber element was gradually pressed down into the reactingurethane rubber composition within half a minute. In a second test, thesectional rubber element was pressed down into the reacting urethanerubber composition 2 minutes after the lifter placed in the castingmould had been covered with the urethane rubber composition. 72 hoursafter casting, an adhesion test was made, and the peel strength wasfound to be 18.6 kN/m in the first test and but 6.2 kN/m in the secondtest. This Example shows that it is of essential importance that therubber bodies to be joined together will contact the urethane rubbercomposition substantially simultaneously.

EXAMPLE 5

Example 2 was repeated with a polyether prepolymer and a differentcuring agent composition. The rubber material in the lifter and in thesectional rubber element was the same as in Example 1. The prepolymerand the curing agent composition were as follows:

Prepolymer

100% by weight polyether prepolymer with about 7.7% of availableisocyanate groups (MDI)

Curing agent

4.6 parts by weight 1,4-butanediol

3.2 parts by weight 1,3-butanediol

72 hours after casting, an adhesion test was carried out, and the peelstrength was found to be 25.4 kN/m. The rupture occured in the styrenebutadiene rubber material, not in the joint between the rubbermaterials.

I claim:
 1. A wear-resistant rubber product adapted to be used as alifter in grinding mills, said product comprising at least twoprefabricated parts, one of said parts including a holder for mountingsaid product, the other said part being a prefabricated vulcanizedwear-resistant rubber body having a profile corresponding to theintended profile of a portion of the rubber product, said parts beinginterconnected with one another by means of a binder, characterized inthat the binder consists essentially of a layer of urethane rubber castand vulcanized in situ between said prefabricated parts, said layerhaving a thickness of at least 2 mm.
 2. A product as claimed in claim 1wherein the part comprising the holder is a worn rubber product of theintended type.
 3. A product as claimed in claim 1 wherein said layer hasa thickness of at least 5 mm.
 4. A product as claimed in claim 1 whereinsaid holder for mounting the product includes means for attaching theproduct to a rotating drum of a grinding mill, and the other said partincludes a lifter portion which has a profile operable as a lifter in agrinding mill.
 5. A wear-resistant lining for installation in a rotatingdrum of a grinding mill comprising at least two prefabricated parts, onesaid part including holder means for attaching it to a rotating drum ofa grinding mill, the other said part being a lifter portion formed ofprefabricated vulcanized wear-resistant rubber, said lifter portionhaving a profile corresponding to the intended profile of a portion ofsaid lining, said parts being interconnected with one another by abinder which consists essentially of a layer of urethane rubber cast andvulcanized in situ between said prefabricated parts, said layer having athickness of at least 2 mm.
 6. A wear-resistant lining as claimed inclaim 5 wherein the one said part is a worn lifter of a grinding mill.